1. Field of the Invention
The present invention relates to an exhaust emission control device of an internal combustion engine, in which a three way catalyst and a catalyst with an NOx absorbing function are disposed in an exhaust gas passage.
2. Description of Related Art
In recent years, a lean burn internal combustion engine which improves fuel economy by running the internal combustion engine at a lean air-fuel ratio has been put to practical use. If the lean bum internal combustion engine is run at the lean air-fuel ratio, a three way catalyst cannot sufficiently purify NOx (nitrogenous substance) in exhaust gas due to its purifying characteristic. To address this problem, an exhaust emission control catalyst device provided with an absorption-type NOx catalyst, which absorbs NOx in exhaust gas while the engine is run at the lean air-fuel ratio and emits and reduces the absorbed NOx while the engine is run at a stoichiometrical air-fuel ratio or a rich air-fuel ratio, has been adopted recently.
The absorption-type NOx catalyst has the characteristics of absorbing NOx in exhaust gas as nitrate (or acid oxide) in an atmosphere with excessive oxygen of exhaust gas, and emitting the absorbed NOx in an atmosphere mainly with excessive carbon monoxide of exhaust gas and reducing the NOx into nitrogen (N2) (carbonate is produced at the same time).
In the internal combustion engine, the three way catalyst is provided at the upstream of an exhaust passage in order to control the exhaust of a large amount of uncombusted HC generated when the engine is cold-started. The three way catalyst, however, ordinarily carries a noble metal (e.g., platinum and rhodium), and the noble metal is oxidized to deteriorate the catalyst performance (heat deterioration) when the exhaust gas becomes a lean atmosphere with a high temperature. Therefore, for the exhaust emission control device in which the catalyst with a three way function is disposed in the exhaust passage, Japanese Patent Provisional Publication No. 5-59935 has proposed an air-fuel ratio control for preventing the heat deterioration by making the exhaust air-fuel ratio stoichiometrical if the catalyst is exposed to a predetermined high temperature in oxide atmosphere.
In the lean burn internal combustion engine, the three way catalyst and the NOx absorption-type catalyst are provided in the exhaust passage. The three way catalyst and the absorption-type NOx catalyst have different deterioration characteristics such as heat deterioration characteristic. For this reason, a technique for effectively controlling the deterioration by using both the three way catalyst and the NOx absorption-type catalyst has not yet been established.
More specifically, the three way catalyst and the NOx absorption-type catalyst differ in their heat-resisting temperatures. Controlling the deterioration of one catalyst may proceed the deterioration of other catalyst. More specifically, the three way catalyst is oxidized and deteriorated when it is exposed to a high temperature and the exhaust gas becomes the lean atmosphere. To address this problem, it is necessary to control the air-fuel ratio in such a manner as to make the exhaust air-fuel ratio stoichiometrical when the three way catalyst is exposed to a high temperature in the oxide atmosphere. The heat-resisting temperature of the absorption-type NOx catalyst, however, is lower than that of the three way catalyst. Thus, if the absorption-type NOx catalyst is exposed to a high temperature and the exhaust gas becomes a stoichiometric atmosphere to reduce CO and THC (the generic name for hydrocarbon substance), absorbing material thereof never becomes carbonate or nitrite (or acid oxide) and is destabilized. Therefore, the absorbing material cannot absorb the NOx by combining with a carrier (deterioration).
Particularly if the flow rate of exhaust gas is high, the oxidization of the three way catalyst advances the deterioration, and if the flow rate of exhaust gas is low, there is a shortage of CO, THC and the like to thereby advance the deterioration of the absorption-type NOx catalyst.
It is therefore an object of the present invention to provide an exhaust emission control device of an internal combustion engine, which can prevent the deterioration such as heat deterioration of the three way catalyst and the absorption-type NOx catalyst to thereby prevent the deterioration of an exhaust gas characteristic and the increase in cost.
The above object can be accomplished by providing an exhaust emission control device of an internal combustion engine, which purifies exhaust gas exhausted from the internal combustion engine; the exhaust emission control device comprising: a catalyst device composed of a three way catalyst for purifying harmful substance in the exhaust gas when an exhaust air-fuel ratio is substantially stoichiometrical and an NOx catalyst having a function of absorbing NOx in the exhaust gas when the air-fuel ratio is closer to a lean air-fuel ratio than to the stoichiometrical air-fuel ratio, the catalyst device being provided in an exhaust passage of the internal combustion engine; catalyst deterioration determination means for determining deterioration state of the catalyst device, the deterioration resulting from at least temperature; and control means for deteriorating exhaust gas components flowing into the catalyst device much more than exhaust gas components flowing into the catalyst device when an air-fuel ratio of a mixture supplied to the internal combustion engine is substantially stoichiometrical, if the catalyst deterioration determination means determines that the catalyst device is in a predetermined deterioration state.
The deterioration of an NOx absorbing function of the NOx catalyst results from at least the destabilization of absorbing material in the NOx catalyst, which is caused by the increase in the temperature. Thus, if the catalyst deterioration determination means determines that the catalyst device is in the predetermined deterioration state, the exhaust gas components flowing into the catalyst device are deteriorated much more than the exhaust gas components flowing into the catalyst device when the air-fuel ratio of the mixture supplied to the internal combustion engine is substantially stoichiometrical. Therefore, CO, H2, NOx, O2, THC and the like are supplied to the absorbing material to thereby stabilize the absorbing material as carbonate, nitrite or acid oxide. This prevents the heat deterioration, extends a heat-resisting life of the catalyst device, and prevents the deterioration of an exhaust gas characteristic and an increase in expense.
In one preferred mode of the present invention, the exhaust emission control device of the internal combustion engine further comprises: catalyst temperature sensing means for sensing or estimating a temperature of the catalyst device; and wherein the catalyst deterioration determination means determines that the catalyst device is in a predetermined deterioration state if a catalyst temperature sensed or estimated by the catalyst temperature sensing means is in excess of a heat-resisting temperature of the catalyst device.
In one preferred mode of the present invention, the exhaust emission control device of the internal combustion engine further comprises: catalyst temperature sensing means for sensing or estimating a temperature of the NOx catalyst; and wherein the catalyst deterioration determination means determines that the catalyst device is in a predetermined deterioration state if a temperature of the NOx catalyst sensed or estimated by the catalyst temperature sensing means is in excess of a heat-resisting temperature of the NOx catalyst.
In one preferred mode of the present invention, the control means prohibits the air-fuel ratio of the mixture supplied to the internal combustion engine from being substantially stoichiometrical in order to deteriorate the exhaust gas components.
In one preferred mode of the present invention, the exhaust emission control device of the internal combustion engine further comprises: catalyst temperature sensing means for sensing or estimating a temperature of the catalyst device; and air-fuel ratio control means for making the air-fuel ratio of the mixture supplied to the internal combustion engine substantially stoichiometrical according to the temperature of the catalyst device sensed or estimated by the catalyst temperature sensing means when a temperature of the three way catalyst is not less than a predetermined temperature; and wherein the catalyst deterioration determination means determines that the catalyst device is in a predetermined deterioration state according to the temperature of the catalyst device sensed or estimated by the catalyst temperature sensing means when the temperature of the NOx catalyst is in excess of a heat-resisting temperature of the NOx catalyst; and the control means prohibits the air-fuel ratio of the mixture supplied to the internal combustion engine from being substantially stoichiometrical when the catalyst deterioration determination means determines that the catalyst device is in a predetermined deterioration state when the air-fuel ratio control means is operating.
In this case, the air-fuel ratio of the mixture supplied to the internal combustion engine is made substantially stoichiometrical when the temperature of the three way catalyst is not less than a predetermined temperature. This lowers the temperature of the exhaust gas and prevents the heat deterioration of the three way catalyst. If the temperature of the NOx catalyst exceeds the heat-resisting temperature of the NOx catalyst during the air-fuel ratio control, the control means prohibits the air-fuel ratio of the mixture supplied to the internal combustion engine from being substantially stoichiometrical. Thus, the exhaust air-fuel ratio is made lean or rich, and CO, H2, NOx, O2, THC and the like are supplied to the NOx absorbing material of the NOx catalyst. This stabilizes the NOx absorbing material, and prevents the heat deterioration of the NOx catalyst.
In one preferred mode of the present invention, the exhaust emission control device of the internal combustion engine further comprises: deterioration parameter finding means for finding a deterioration parameter of the three way catalyst and a deterioration parameter of the NOx catalyst; first deterioration determination means for determining that the three way catalyst is in a predetermined deterioration state when the deterioration parameter of the three way catalyst found by the deterioration parameter finding means is in excess of a reference value that is preset for the three way catalyst; and second deterioration determination means for determining that the NOx catalyst is in a predetermined deterioration state when the deterioration parameter of the NOx catalyst found by the deterioration parameter finding means is in excess of a reference value that is preset for the NOx catalyst; and wherein if the first deterioration determination means determines that the three way catalyst is in a predetermined deterioration state, the control means prohibits the air-fuel ratio of the mixture supplied to the internal combustion engine from being the lean air-fuel ratio, and if the second deterioration determination means determines that the NOx catalyst is in a predetermined deterioration state, the control means prohibits the air-fuel ratio of the mixture supplied to the internal combustion engine from being substantially stoichiometrical.
In this preferred mode, one catalyst, which has a higher deterioration parameter, i.e., one catalyst, which is deteriorated more easily, is given priority in the prevention of the deterioration. If both catalysts have high deterioration parameters, the internal combustion engine is prohibited from operating at the lean air-fuel ratio and the substantially stoichiometrical air-fuel ratio, and this prevents the deterioration resulting from the oxidization of the three way catalyst and the deterioration resulting from the destabilization of the absorbing material in the NOx catalyst. Moreover, the operation of the internal combustion engine at the rich air-fuel ratio can be reduced to the minimum level, and this controls the deterioration of the fuel economy.
In one preferred mode of the present invention, the deterioration parameter finding means finds the deterioration parameter of the three way catalyst by using an deterioration index corresponding to a temperature of the three way catalyst, and finds the deterioration index of the NOx catalyst by using a deterioration index corresponding to a temperature of the NOx catalyst.
In one preferred mode of the present invention, the deterioration index corresponding to the temperature of the three way catalyst indicates that the higher the temperature of the three way catalyst is, the larger the deterioration degree of the three way catalyst is, and the deterioration index corresponding to the temperature of the NOx catalyst indicates that the higher the temperature of the NOx catalyst is, the larger the deterioration degree of the NOx catalyst is.
In one preferred mode of the present invention, the deterioration index finding means finds the deterioration index of the three way catalyst by using at least either one of an deterioration index corresponding to a flow rate of exhaust gas flowing into the three way catalyst and an deterioration index corresponding to components of exhaust gas flowing into the three way catalyst and a deterioration index corresponding to a temperature of the three way catalyst, and finds the deterioration index of the NOx catalyst by using at least either one of a deterioration index corresponding to a flow rate of the NOx catalyst and a deterioration index corresponding to components of exhaust gas flowing into the NOx catalyst and a deterioration index corresponding to a temperature of the NOx catalyst.
It is therefore possible to determine the degree of the deterioration according to factors such as the temperature, the flow rate of the exhaust gas and the exhaust gas components, which are closely related to the deterioration of the catalyst device. This enables the correct determination about the deterioration state and the proper treatment for the determined deterioration state.
In one preferred mode of the present invention, the deterioration index corresponding to the flow rate of exhaust gas flowing into the three way catalyst indicates that the hither the flow rate of the exhaust gas is, the larger the deterioration degree of the three way catalyst is; the deterioration index corresponding to the components of the exhaust gas flowing into the three way catalyst indicates that the leaner the exhaust air-fuel ratio is, the larger the deterioration degree of the three way catalyst is; the deterioration index corresponding to the temperature of the three way catalyst indicates that the higher the temperature of the three way catalyst is, the deterioration degree of the three way catalyst is; the deterioration index corresponding to the flow rate of the exhaust gas flowing into the NOx catalyst indicates that the lower the flow rate of the exhaust gas is, the larger the deterioration degree of the NOx catalyst is; the deterioration index corresponding to the components of the exhaust gas flowing into the NOx catalyst indicates that the closer the exhaust air-fuel ratio is to the stoichiometrical air-fuel ratio, the larger the deterioration degree of the NOx catalyst is; and the deterioration index corresponding to the temperature of the NOx catalyst indicates that the higher the temperature of the NOx catalyst is, the larger the deterioration degree of the NOx catalyst is.
The above object can also be accomplished by providing an exhaust emission control device of an internal combustion engine, which purifies exhaust gas exhausted from the internal combustion engine, the exhaust emission control device comprising: a catalyst device composed of a three way catalyst for purifying harmful substance in the exhaust gas when an exhaust air-fuel ratio is substantially stoichiometrical and an NOx catalyst having a function of absorbing NOx in the exhaust gas when the air-fuel ratio is closer to a lean air-fuel ratio than to the stoichiometrical air-fuel ratio, the catalyst device being provided in an exhaust passage of the internal combustion engine; operating state setting means for selecting and setting an operating state from a plurality of operating states including a lean air-fuel ratio operating state wherein a mixture supplied to the internal combustion engine is closer to a lean air-fuel ratio than to a stoichiometrical air-fuel ratio and a rich air-fuel operating state wherein a mixture supplied to the internal combustion engine is closer to a rich air-fuel ratio than to the stoichiometrical air-fuel ratio according to a parameter correlating to a temperature of the catalyst device and a parameter correlating to a flow rate of exhaust gas flowing into the catalyst device; and control means for controlling the mixture of the internal combustion engine according to the operating state set by the operating state setting means.
This exhaust emission control device selects a suitable operating state according to the temperature and the flow rate of the exhaust gas in conformance with the deterioration characteristics of the catalysts to thereby reduce the deterioration of the fuel economy and control the deterioration of both the three way catalyst and the NOx catalyst.
In one preferred mode of the present invention, the operating state setting means sets an air-fuel ratio operating state for the three way catalyst according the parameter correlating to the temperature of the three way catalyst and the parameter correlating to the flow rate of the exhaust gas flowing into the three way catalyst, and sets an air-fuel ratio operating state for the NOx catalyst according to the parameter correlating to the temperature of the NOx catalyst and the parameter correlating to the flow rate of the exhaust gas flowing into the NOx catalyst, and if these two air-fuel ratio operating states are different, the operating state setting means selects a leaner air-fuel ratio operating state.
In one preferred mode of the present invention, a temperature region of the NOx catalyst, where the air-fuel ratio operating state for the NOx catalyst selected by the operating state setting means switches to a richer air-fuel ratio operating state, is shifted to a higher temperature side as the flow rate of the exhaust gas is decreased.
The absorbing material of the NOx catalyst is stabilized as nitrite (or acid oxide) at the lean exhaust air-fuel ratio. For this reason, it is possible to prevent the emission of oxygen or NOx and maintain the stable state of the absorbing material by switching the operating state to the richer air-fuel ratio operating state at the high temperature.
In one preferred mode of the present invention, a temperature region of the NOx catalyst, where the air-fuel ratio operating state for the NOx catalyst selected by the operating state setting means switches to a leaner air-fuel ratio operating state, is shifted to a lower temperature side as the flow rate of the exhaust gas is decreased.
The absorbing material of the NOx catalyst is stabilized as carbonate at the rich exhaust air-fuel ratio. For this reason, it is possible to prevent the emission of carbonate and maintain the stable state of the absorbing material by switching the operating state to the leaner air-fuel ratio operating state at the low temperature.